Atomic Layer Deposition (“ALD”), also known as Atomic Layer Epitaxy (“ALE”), is a method of depositing thin films onto a substrate that involves sequential and alternating self-saturating surface reactions. The ALD process is described in U.S. Pat. No. 4,058,430 of Suntola et al., which is incorporated herein by reference. ALD offers several benefits over other thin film deposition methods, such as Physical Vapor Deposition (“PVD”) (e.g., evaporation or sputtering) and Chemical Vapor Deposition (“CVD”), which are well known to those skilled in the art, as described in Atomic Layer Epitaxy (T. Suntola and M. Simpson, eds., Blackie and Son Ltd., Glasgow, 1990). ALD methods have been proposed for use in depositing thin films on semiconductor wafer substrates, to achieve desired step coverage and physical properties needed for next-generation integrated circuits.
Successful ALD growth requires the sequential introduction of two or more precursor vapors into a reaction space around the substrate surface. Typically, ALD is performed at elevated temperatures and reduced pressures. For example, the reaction space may be heated to between 150° C. and 600° C., and operated at a pressure of between 0.1 mbar and 50 mbar. Even at such high temperatures and low operating pressures, pulses of precursor vapors are not delta functions, meaning they have a substantial rise and decay time. Sequential pulses of precursor vapors will overlap if the second pulse is started before the first pulse is completely decayed, i.e., before excess first precursor vapor is substantially eliminated from the reaction space. If substantial amounts of the different precursor vapors are present in the reaction space at the same time, then non-ALD growth can occur, which can generate particles or non-uniform film thickness. To prevent this problem, the pulses of precursor vapor are separated by a purge interval during which the reaction space is purged of excess amounts of the first precursor vapor. During the purge interval, the reaction chamber is purged by flushing the reaction chamber with an inert gas, application of a vacuum, pumping, suction, or some combination thereof.
The ALD reaction space is typically bounded by a reaction chamber, which is fed by one or more precursor material delivery systems (also called “precursor sources”). The size of the reaction space is affected by the dimensions of the reaction chamber needed to accommodate the substrate. Some reaction chambers are large enough to fit multiple substrates for batch processing. However, the increased volume of the reaction space in a batch processing system may require increased precursor pulse durations and purge intervals.
To prevent overlap of the precursor pulses and to form thin films of relatively uniform thickness, an ALD process may require purge intervals that are ten times longer than the duration of precursor vapor pulses. For example, a thin film deposition process may include thousands of precursor vapor pulses of 50 ms duration alternating with purge intervals of 500 ms duration. Long purge intervals increase processing time, which can substantially reduce the overall efficiency of an ALD reactor. The present inventors have recognized that reducing the rise and decay times also reduces the overall time required for precursor pulse and purge without causing non-ALD growth, thereby improving the throughput of the ALD reactor.
A precursor material delivery system may typically include one or more diaphragm valves positioned in a flow path of the system, for preparing and dispensing one or more precursor vapors. The precursor vapors are pulsed into the reaction chamber by opening and closing the appropriate diaphragm valves in the precursor delivery system. Diaphragm valves can also be used for controlling the flow of inert gases and other materials into and out of the ALD reactor. Known diaphragm valves commonly have an actuator for opening and closing a flexible diaphragm against a valve seat. When the diaphragm is in the open position, the precursor vapor is allowed to pass through a valve passage and enter the reaction chamber. When closed, the diaphragm blocks the valve passage and prevents the precursor vapor from entering the reaction chamber. Because ALD processing can require many thousands of cycles of precursor pulse and purge for forming a film on a single workpiece, valves used in an ALD system should have very high durability and be able to perform millions of cycles without failure.
Hydraulic and pneumatic actuators typically include dynamic seals that can fail under the high temperatures and large number of cycles required for delivery of precursor gases and purge gases in an ALD system.
Solenoid type actuators are desirable because they typically have a faster response time than pneumatic and hydraulic actuators, and are capable of a large number of open-close cycles. However, solenoid actuators generate heat when electric current is applied and, like hydraulic and pneumatic valves, solenoid actuated valves can fail when exposed to the high temperatures required for maintaining some precursor materials in vapor form. Heat can degrade the insulation around the solenoid windings, resulting in electrical shorting between windings and failure of the solenoid coil. It can also melt a plastic bobbin around which the solenoid coil is wound. The present inventors have recognized that active cooling of the actuator to avoid heat-related failure tends to also draw heat from the diaphragm, valve seat, and walls of the valve passage, which can cause the precursor material to condense or solidify in the valve passage. Condensation and buildup of precursor material on the diaphragm and valve seat can cause the valve to leak or clog, leading to undesirable non-ALD growth and particles in the reaction chamber.
For successful ALD processing, precursor gases are typically delivered to the reaction chamber at temperatures in excess of 100° C. and often between 200° C. and 300° C., particularly the varieties of precursor materials used for forming thin films on semiconductor substrates. With a conventional diaphragm valve, a significant amount of heat is conducted from the flow path through the valve, where it dissipates to the surrounding environment. Heat dissipation through the valve can result in cooling of the flow path and the associated condensation problems discussed above. To avoid condensation, the flow path may be heated, as described, for instance, in U.S. Provisional Patent Application No. 60/410,067 filed Sep. 11, 2002, titled “Precursor Material Delivery System for Atomic Layer Deposition,” which is owned by the assignee of the present invention and incorporated herein by reference. However, heating the flow path may tend to contribute to overheating of the actuators in conventional diaphragm valves. The present inventors have recognized a need for an improved diaphragm valve in which the valve passage, diaphragm, and valve seat can be kept hot enough to prevent the precursor vapor from condensing (typically in the range of 130° C. to 260° C. or hotter), without overheating the valve actuator.
U.S. Pat. No. 5,326,078 of Kimura, U.S. Pat. No. 6,116,267 of Suzuki et al., and U.S. Pat. No. 6,508,453 of Mamyo describe known diaphragm valves for controlling the flow of high temperature gases for semiconductor manufacturing.
The present inventors have recognized that a need remains for a valve in which the diaphragm is kept at a temperature sufficient to prevent condensation of ALD precursor materials while not exceeding the temperature limits of the actuator. The inventors have also recognized a need for a durable valve that transitions from an open position to a closed position more quickly than prior art valves.